This invention relates in general to apparatus and methods for anaerobic digestion of sludges produced in water treatment processes cur the like. More particularly, it relates to systems for removing heavy solids that settle to the bottom of digester vessels and controlling the accumulation of light solids that rise to the surface of such digesters and may exhibit a tendency to form foam, froth, or emulsions.
A wide variety of sludges are treated in anaerobic digesters. Historically, anaerobic sludge digestion has been used for stabilizing primary clarifier sludges. More recently, it has been applied to biological sludges produced by activated sludge or trickling filter processes, and to sludge mixtures containing significant industrial waste contributions. Municipal sewage sludges typically have significant quantities of manufactured products ranging from fibrous toiletry products to plastics, or other synthetic products ranging from long filaments to sheets of bulky materials. Modern biological secondary treatment plants often produce significant quantities of aerobic microorganisms that are not easily decomposed in an anaerobic digester. In almost all cases, the raw sludge sent to an anaerobic digester is a very diverse and complex mixture of materials ranging from simple inert silt, sand, and soil particles to very complex organic molecules and particles.
During anaerobic digestion, materials are segregated in a variety of ways. Some relatively light materials entrap rising gas bubbles and are transported to the liquid surface in the digester. Similarly, some of the microscopic biomass in raw sludge retains microscopic bubbles and is transported to the surface. Other materials having a specific gravity less than the digester liquid in which they are suspended rise through natural buoyancy. At the surface, these materials can create a foam or froth layer, in which little digestion occurs. The fibrous and string-type materials typically found in municipal sludges tend to become entangled with each other and accumulate in a scum layer at the liquid surface. This scum layer can include mats and ropes that hinder the digestion process.
In contrast, inert heavy materials settle to the bottom of the digester. Conventional American anaerobic digesters have a large relatively level floor, where the heavy materials remain relatively stationary.
Recently, German-style tall cylindrical or egg-shaped digesters have started to be used in the United States and Canada. Some of these digesters are provided with a vertical draft tube located on the central axis of the digester. The draft tube includes motor-operated pumps or jets that provide a mechanism for mixing the liquid in the digester by pumping large quantities of anaerobic digester liquid from the bottom of the digester to the top, and vice versa.
Heavy materials are typically removed from the floor through a discharge pipe extending from the bottom of the digester. In German-style digesters, the discharge pipe for removing heavy materials from the bottom of the digester leads to an externally mounted discharge chamber located near the top of the digester. The discharge chamber is located near the top of the digester so the discharge pipe can act as a standpipe to control the liquid level in the digester. The typical discharge standpipe terminates in an adjustable telescoping valve in the form of a sliding pipe that can be raised or lowered, typically within a 3- to 4-foot range. This discharge system requires frequent flushing and/or pressurized back-flushing to clear clogs in the discharge pipe or at its mouth near the bottom of the digester. The difficulties in moving heavy bottom material upward through a relatively small diameter pipe (which usually has sloped zones and several fittings or elbows) has proven difficult and troublesome in egg-shaped digesters recently installed in the U.S. and Canada.
Foam control in tall cylindrical and egg-shaped digesters varies. When the digester includes a draft tube with motor-operated pumps, foam control generally depends upon the liquid drawn up through the draft tube splashing over the top of the foam to knock it down, or drawing the foam into the draft tube for transport to the bottom of the vessel. Some German facilities without a central draft tube have incorporated a mechanical mixer that stirs the liquid surface and also breaks up scum.
Other systems that do not utilize a draft tube have provided a spray nozzle or nozzles above the liquid surface to minimize foam formation. These nozzles are pointed at an angle at the liquid surface. In some applications, spray systems with moderate discharge velocities and multiple nozzles have been reasonably effective. However, the effectiveness is directly related to the nature and quantity of the raw sludge loaded to the system. The closer a system gets to the design load or maximum sustained load, the more difficult it may become to control foam, froth, and/or emulsions. Furthermore, raw sludges that contain large quantities of waste-activated sludge can create difficult and special problems related to the fibrous microorganisms that are created in the process. The typical German and American foam-control systems utilized to date have not proven effective with these types of sludges.
Tall cylindrical and egg-shaped digesters typically have a scum discharge funnel located near the top of the digester. The funnel has a vertical mouth that faces the central vertical axis of the digester. When the level of liquid in the digester is high, significant quantities of digester surface material are drawn into the mouth of the funnel. As material is withdrawn and the liquid level drops, the cross-sectional area of the mouth under the surface level decreases until the bottom of the funnel mouth is reached, at which point scum discharge stops. This system was developed to allow large chunks of scum or other liquid surface accumulations to be drawn out of the vessel. Unfortunately, the system is not very effective.
An alternate method of scum removal uses one or more doors that can be opened on the side of the digester to allow surface materials either to spill out or to be manually raked out of the digester by hand. This method creates significant health and safety risks.
New apparatus and methods are needed to provide improved, more effective, and more reliable means and methods for removing heavy materials from the bottom of a digester; for foam, froth, and emulsions suppression and control; and for removing fibrous and/or matted top materials from the top of a digester.